1. Field of the Invention
The present invention relates to signal transmission circuits and display apparatus, and it particularly relates to a signal transmission circuit and display apparatus used when inspection signals are outputted therefrom.
2. Description of the Related Art
In recent years, the liquid crystal display (LCD) have been widely used as display apparatus for various electric machinery and apparatus, but the display considered promising as a next-generation flat display panel is the organic EL (Electro Luminescence) display. The display using the active matrix system as a display method for such displays is called the active matrix display. In the active matrix display, a multiplicity of pixels are vertically and horizontally disposed in a matrix, and a switching element is provided for each pixel. The group of pixels in the matrix is sequentially selected by the signal line drive circuit that drives the signal lines transmitting the luminance data and by the scanning line drive circuit that drives the scanning lines, so as to write the data thereto. For example, shift registers are used in these signal line drive circuit and scanning line drive circuit.
As the active matrix displays like this come to be widely used, there is a growing demand for the capability to switch the direction of data writing to the pixels thereof. For example, the way these displays are incorporated and implemented into the end products, which are mostly electrical equipment, varies with the type of the electrical equipment, and it is necessary to switch the direction of data writing according to how the display is to be incorporated.
Moreover, in various cameras with a built-in display, switching of data writing direction is required, for example, between normal display for the shots of normal objects and mirror-image display for the shots of the user himself/herself. In this case, there is also required the drive circuits that can switch the direction of data writing.
It is to meet these requirements that signal line drive circuits and scanning line drive circuits, which employ shift registers capable of transferring data in both directions, have been developed. Such circuits are disclosed, for example, in Japanese Patent Application Laid-Open No. Hei 10-74060.
The patent specifications, such as Japanese Patent Application Laid-Open No. 2000-131708, discloses a technique for checking signals outputted from the final stage of a shift register of a signal line drive circuit or a scanning line drive circuit in order to inspect for the operation state of the signal line drive circuits or the scanning line drive circuits of the matrix type display as mentioned above. In this way, it is possible to detect deterioration of transistors by checking the signals outputted from the final stage of a shift register.
Such inspection, however, has disadvantages in that the longer the distance between the final stage of a shift register and the connector pin, which is the inspection signal output terminal, the greater the distortion of the outputted signals will be due to the effect of wiring load. This disadvantage causes a problem where the inspection cannot be performed with desired accuracy. Especially with a signal line drive circuit or a scanning line drive circuit capable of switching the direction of data writing, signals from both the first stage and the final stage of the shift register need to be taken out as inspection signals. Normally, signal line drive circuits and scanning line drive circuits are disposed in the periphery of a display area, so that the first stages and the final stages of the shift registers are each positioned at a distance determined by the width and height of the display area. Accordingly, if the outputs from both the first stages and the final stages of shift registers of the signal line drive circuits and the scanning line drive circuits are to be taken out and received with accuracy, circuit design capable of properly correcting the distortion of the output signals needs to be carried out in light of the layout of these shift resistors and the output terminals thereof.